Broadband Near-Infrared Luminescence in Lead Germanate Glass Triply Doped with Yb3+/Er3+/Tm3+

Abstract

This paper deals with broadband near-infrared luminescence properties of lead germanate glass triply doped with Yb³⁺/Er³⁺/Tm³⁺. Samples were excited at 800 nm and 975 nm. Their emission intensities and lifetimes depend significantly on Er³⁺ and Tm³⁺ concentrations. For samples excited at 800 nm, broadband emissions corresponding to the overlapped ³H₄ → ³F₄ (Tm³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transitions centered at 1.45 µm and 1.5 µm was identified. Measurements of decay curves confirm reduction of ³H₄ (Tm³⁺), ²F_5/2 (Yb³⁺) and ⁴I_13/2 (Er³⁺) luminescence lifetimes and the presence of energy-transfer processes. The maximal spectral bandwidth equal to 269 nm for the ³F₄ → ³H₆ transition of Tm³⁺ suggests that our glass co-doped with Yb³⁺/Er³⁺/Tm³⁺ is a good candidate for broadband near-infrared emission. The energy transfer from ⁴I_13/2 (Er³⁺) to ³F₄ (Tm³⁺) and cross-relaxation processes are responsible for the enhancement of broadband luminescence near 1.8 µm attributed to the ³F₄ → ³H₆ transition of thulium ions in lead germanate glass under excitation of Yb³⁺ ions at 975 nm.

1. Introduction

Lanthanide triply doped inorganic glass is an excellent candidate for ultra-wide near-infrared (NIR) luminescence covering the 1200–2100 nm spectral range [1]. Systematic studies demonstrate that bands of selected lanthanide ions located in the NIR region are quite well overlapped, making an important contribution to broadband luminescence. Several different glasses triply doped with lanthanide ions were proposed as efficient systems emitting NIR radiation. There are glass systems containing Nd³⁺/Er³⁺/Tm³⁺ [2], Nd³⁺/Er³⁺/Pr³⁺ [3], Yb³⁺/Er³⁺/Pr³⁺ [4], and Yb³⁺/Ce³⁺/Er³⁺ [5], important for the optical telecommunication window (1200–1650 nm) as well as Yb³⁺/Tm³⁺/Ho³⁺ [6] and Yb³⁺/Er³⁺/Ho³⁺ [7] for NIR laser sources at about 2 µm. From the experimental tests of different glass systems, it can be concluded that low-phonon inorganic glasses triply doped with lanthanide ions are promising for numerous applications in the field of infrared photonics and laser technology such as optical telecommunications, broadband near-infrared fiber amplifiers, solid-state laser sources, and other optoelectronic devices.

Among fully amorphous systems and glass–ceramic materials, the glasses with suitable Yb³⁺/Er³⁺/Tm³⁺ ion combination are interesting mainly for two purposes. Three simultaneously observed emission bands assigned to the ¹G₄ → ³H₆ transition of Tm³⁺ (blue band), the ²H_11/2,⁴S_3/2 → ⁴I_15/2 (green band), and ⁴F_9/2 → ⁴I_15/2 (red band) transitions of Er³⁺ under direct excitation of Yb³⁺ at 975 nm favor the generation of white light through a well-known mechanism of up-conversion process. To obtain white emission, the concentrations of lanthanide ions (Yb³⁺, Er³⁺, Tm³⁺) and their relative molar ratios should be optimized, and the pump power of the up-conversion process should also not be ignored. Thus, white up-conversion luminescence of Yb³⁺/Er³⁺/Tm³⁺ ions in glass [8,9,10] and glass–ceramic materials containing fluoride nanocrystals YF₃ [11,12] were successfully observed. These effects were also examined for glass with silver nanoparticles [13,14].

Alternatively to the white up-conversion process, glass with Yb³⁺/Er³⁺/Tm³⁺ ions is also attractive for near-infrared radiation. Three near-infrared emission bands related to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ at 1.5 µm, the ³H₄ → ³F₄ (1.45 µm), and ³F₄ → ³H₆ (1.8 µm) transitions of Tm³⁺ can be observed under excitation at 800 nm or 975 nm. However, these phenomena have rarely been examined. For systems with Yb³⁺/Er³⁺/Tm³⁺, near-infrared luminescence properties were limited to multicomponent TeO₂–ZnO–WO₃–TiO₂–Na₂O glass [15] and oxyfluoride silicate glass ceramics containing nanocrystals PbF₂ [16,17].

This paper concerns broadband near-infrared luminescence in lead germanate glass triply doped with Yb³⁺/Er³⁺/Tm³⁺. To the best of our knowledge, these aspects have not been studied before. In general, lead germanate-based glass doped with lanthanide ions and its structure and optical properties are well documented in the literature [18,19,20,21,22]. They are an alternative candidate to tellurite glass for nonlinear fiber applications [23]. In recent years, luminescence properties of lead germanate glass singly doped with Yb³⁺ [24,25,26], Er³⁺ [27,28,29], and Tm³⁺ [30,31] have been well presented and discussed. Special attention has been paid to lead germanate glass co-doped with Yb³⁺/Er³⁺ [32,33,34] and Yb³⁺/Tm³⁺ [35,36], and their up-conversion luminescence processes. Further experimental studies revealed that lead germanate glass triply doped with Yb³⁺/Tm³⁺/Ho³⁺ [37,38,39], Yb³⁺/Tm³⁺/Nd³⁺ [40], and Yb³⁺/Tm³⁺/Er³⁺ [41] ions are promising materials for up-conversion luminescence applications.

2. Materials and Methods

Lead germanate glass triply doped with rare earths with chemical formula [mol%] 45PbO-45GeO₂-(5-x-y)Ga₂O₃-5Yb₂O₃-xTm₂O₃-yEr₂O₃ (x and y = 0, 0.5, 1.5) were prepared. Glass codes are as follows: 0.5 Tm-0.5 Er; 0.5 Tm-1.5 Er; 1.5Tm-0.5Er; and 1.5Tm-1.5Er. They were compared to glass samples co-doped with Yb³⁺/Tm³⁺ referred to as 0.5 Tm and 1.5 Tm, respectively. Precursor metal oxides of high purity (99.99%) were mixed in an agate ball mill. The batch of the starting reagents was placed into a ceramic crucible and the melt was directly poured onto a preheated steel plate. Melting temperature and time are as follows: T = 1100 °C, t = 0.5 h. To reduce the internal stresses, the obtained glass was annealed below the glass transition temperature. For the optical measurements, the glass samples were adequately cut and polished to achieve excellent transparency. Eventually, glass samples with dimensions of 10 × 10 mm and thickness of 2 mm were obtained. Luminescence spectra measurements were carried out using a laser system, which consists of an optical parametric oscillator coupled with Nd:YAG (Continuum Surelite OPO and SLI-10 Nd:YAG laser, Santa Clara, CA, USA), 1 m double grating monochromator, a photomultiplier, boxcar integrator (Stanford SRS250), and oscilloscope (Tektronix model TDS3052, two-channel color digital phosphor oscilloscope, 500 MHz, Tektronix Inc., Beaverton, OR, USA). The investigated glass was mounted in a sample holder and an excitation beam was directed on the sample side edge from a distance of 10 cm. To avoid the signal saturation, the excitation beam was directed perpendicular to a monochromator aperture and the laser spot on the sample was no higher than 2 mm. The resulting signal was collected from the greatest volume of the glass samples using a convex 75 mm lens. The excitation laser power both for 800 nm and 975 nm was set at 450 mW. Resolution for luminescence spectra measurements was ±0.2 nm. Decays were registered with an accuracy of ±2 µs. For the luminescence decay curve measurements, the excitation pulse laser duration was 4 ns, and the pulse energies depending on the applied wavelengths were between 20–40 mJ. To record the NIR transients the InGaAs Hamamatsu and a cooled InSb Janson J10D detectors were used. Moreover, Schott optical long-pass filters RG780, RG850, and RG1000 were employed. The experimental lifetimes of the ³F₄ (Tm³⁺), ³H₄ (Tm³⁺), ⁴I_13/2 (Er³⁺), and ²F_5/2 (Yb³⁺) luminescent levels have been measured at the following adequate wavelengths: 1780 nm, 815 nm, 1530 nm, and 982 nm.

3. Results and Discussion

Luminescence properties of lead germanate glass triply doped with Yb³⁺/Er³⁺/Tm³⁺ were examined in two NIR ranges, where emission bands of Tm³⁺ and/or Er³⁺ occur. The first spectral region (1200–1675 nm) is associated with the so-called telecommunication window. Several inorganic glasses were tested to achieve optical amplification covering the S-band (1460–1530 nm), C-band (1530–1565 nm), L-band (1565–1625 nm), and U-band (1625–1675 nm). In this near-infrared range, the spectrum consists of luminescence bands due to characteristic ³H₄ → ³F₄ (Tm³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) electronic transitions, which are relevant for the design of S-band and C+L-band amplifiers [42]. The second spectral region discussed here deals with a broadband NIR emission at 1800 nm corresponding to ³F₄ → ³H₆ transition of Tm³⁺.

Figure 1 presents luminescence spectra for lead germanate glass triply doped with Yb³⁺/Er³⁺/Tm³⁺. The spectra were compared to those for samples co-doped with Yb³⁺/Tm³⁺. To understand the energy-transfer processes, their mechanisms, and the interactions between lanthanide ions, the lead germanate glass with various Tm³⁺ and Er³⁺ concentrations was excited at 800 nm and 975 nm.

Spectra measured in the 1330–1700 nm range were normalized to compare their emission profiles and bandwidth referred to as full width at half maximum (FWHM). For glass samples excited at 800 nm, the spectra showed emission bands centered at about 1450 nm and 1530 nm, which are assigned to the ³H₄ → ³F₄ (Tm³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transitions of lanthanides. Owing to some excellent papers published previously [43,44,45,46], the shoulder near 1650 nm in sample 1.5 Tm–0.5 Er belongs to the short-wavelength tail of the emission due to the ³F₄ → ³H₆ transition of Tm³⁺. The relative integrated intensities of NIR emission bands depend on Er³⁺ and Tm³⁺ concentrations. In particular, the changes in emission profiles and bandwidths are clearly visible for glass samples with higher Tm³⁺ (1.5 mol%) concentration. In contrast to glass samples with low (0.5 mol%) concentration, the intensity of the NIR emission band due to the ³H₄ → ³F₄ transition of Tm³⁺ decreases with increasing Er³⁺ concentration. The emission bandwidth for glass samples assigned as 1.5 Tm–0.5 Er is close to 130 nm. It is in a good agreement with the value of FWHM equal to 138 nm, which was obtained for similar germanate glass co-doped with Er³⁺/Tm³⁺ [47]. For glass sample 1.5 Tm–0.5 Er, emissions of Tm³⁺ and Er³⁺ ions are quite well overlapped giving contribution to broadband near-infrared radiation related to the S+C+L-bands of the optical telecommunication. These effects are not observed when glass samples triply doped with Yb³⁺/Er³⁺/Tm³⁺ ions were excited at 975 nm. In this case, the emission band with its typical profile for the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions was measured under direct excitation of Yb³⁺ ions. The values of FWHM are about 55 nm and depend slightly on Tm³⁺ and Er³⁺ concentrations.

According to the partial energy level diagram presented in Figure 2, several energy-transfer mechanisms for the studied glass excited at 800 nm and 975 nm are proposed. When a glass sample is excited directly at 800 nm, both the ³H₄ (Tm³⁺) and ⁴I_9/2 (Er³⁺) states are simultaneously populated from their ground states. Part of the excitation energy relaxes radiatively from the ³H₄ state and contributes greatly to near-infrared emissions at about 1.45 µm and 1.8 µm, which are associated with ³H₄ → ³F₄ and ³F₄ → ³H₆ transitions of Tm³⁺. At the same time, the ³H₄ state is quite efficiently depopulated by the nearly resonant energy-transfer process to the ⁴I_9/2 state of Er³⁺ and non-resonant energy-transfer process to the ²F_5/2 state of Yb³⁺. Thus, near-infrared luminescence at about 1 µm due to the ²F_5/2 → ²F_7/2 transition of Yb³⁺ can be observed (not presented here). Additionally, energy is transferred non-radiatively from the ²F_5/2 (Yb³⁺) to the ⁴I_11/2 (Er³⁺) and ³H₅ (Tm³⁺) states of lanthanides. The presence of both phonon-assisted energy-transfer processes ³H₄ (Tm³⁺) → ²F_5/2 (Yb³⁺) and ²F_5/2 (Yb³⁺) → ³H₄ (Tm³⁺) was confirmed in Yb³⁺/Tm³⁺ co-doped tellurite glasses [48].

During direct excitation of glass sample at 800 nm, depopulation of the ⁴I_9/2 (Er³⁺) state is very fast by multiphonon relaxation via ⁴I_11/2 state to the ⁴I_13/2 state, from which near-infrared emission at 1.5 µm assigned to ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ occurs. The first excited ⁴I_13/2 state of erbium is also depopulated non-radiatively and part of the excitation energy is transferred to thulium due to the following nearly resonant energy-transfer process ⁴I_13/2 (Er³⁺) → ³F₄ (Tm³⁺). Consequently, the enhanced NIR emission at 1.8 µm due to the ³F₄ → ³H₆ transition of Tm³⁺ can be observed. However, the most important non-radiative transitions that contribute to quenching (at 1.45 µm) and enhancing (near 1.8 µm) the near-infrared luminescence of Tm³⁺ are related to two cross-relaxation processes [45]: [³H₄ (Tm³⁺) + ³H₆ (Tm³⁺)] → [(³F₄ (Tm³⁺) + ³F₄ (Tm³⁺)] and [³H₄ (Tm³⁺) + ⁴I_15/2 (Er³⁺)] → [³F₄ (Tm³⁺) + ⁴I_13/2 (Er³⁺)].

When a glass sample is excited at 975 nm the ²F_5/2 state of Yb³⁺ ions is quite well populated and then the excitation energy relaxes non-radiatively to the ⁴I_11/2 (Er³⁺) and ³H₅ (Tm³⁺) states by nearly resonant and non-resonant (phonon-assisted) energy-transfer process, respectively. In the next step, multiphonon relaxation contributes to the efficient population of lower-energy ⁴I_13/2 (Er³⁺) and ³F₄ (Tm³⁺) states. Consequently, near-infrared emission bands at 1.5 µm and 1.8 µm corresponding to the ⁴I_13/2 → ⁴I_15/2 (Er³⁺) and ³F₄ → ³H₆ (Tm³⁺) transitions of lanthanides are observed under excitation of Yb³⁺ ions at 975 nm.

Figure 3 shows luminescence decays from the ³H₄ (Tm³⁺) and ²F_5/2 (Yb³⁺) states, which were measured for glass samples excited at 800 nm and 975 nm, respectively. All decay curves exhibit a slight deviation from the single-exponential function.

For luminescence decays measured under 800 nm excitation, the curves for both simultaneously and resonantly excited states ³H₄ (Tm³⁺) and ⁴I_9/2 (Er³⁺) should be observed, because the positions of these states on the energy level diagram are nearly the same. However, it is experimentally proved that the ⁴F_9/2 lifetime of Er³⁺ ions is one or two magnitudes of order lower than the ³H₄ lifetime of Tm³⁺ ions due to the very fast non-radiative process to the lower-lying ⁴I_11/2 (Er³⁺) state by the efficient multiphonon relaxation, and as a result its contribution to the overall luminescence decay is negligible [49]. Thus, decays measured from the ³H₄ (Tm³⁺) state should be reduced. Luminescence lifetimes calculated based on decay curves should be shortened due to the depopulation of ³H₄, the state of Tm³⁺ ion, and the presence of the energy-transfer process ³H₄ (Tm³⁺) → ⁴I_9/2 (Er³⁺). Luminescence decay analysis confirms this hypothesis. The results are shown in Table 1.

For glass samples with low Tm³⁺ concentration (0.5 mol%), the measured ³H₄ lifetime decreased from 128 µs (0.5 Tm) to 103 µs (0.5 Tm − 0.5 Er) and 53 µs (0.5 Tm − 1.5 Er) in the presence of Er³⁺ ions, suggesting an efficient energy-transfer process from a ³H₄ (Tm³⁺) state to a ⁴I_9/2 (Er³⁺) state. The reduction of luminescence lifetime is considerably lower for glass samples with relatively higher Tm³⁺ concentration (1.5 mol%). Similar effects were also obtained for decays from the ²F_5/2 excited state of Yb³⁺. The measured ²F_5/2 luminescence lifetime of Yb³⁺ is reduced from 201 µs (0.5 Tm) to 165 µs (0.5 Tm − 0.5 Er) and 109 µs (0.5 Tm − 1.5 Er) in the presence of Er³⁺ ions, whereas its value 131 ± 3 µs is nearly unchanged for glass samples with a higher Tm³⁺ concentration. The same situation was observed during measurements of luminescence decays from the ⁴I_13/2 excited state of Er³⁺ ions. For lead germanate glass triply doped with Yb³⁺/Er³⁺/Tm³⁺ ions, the ⁴I_13/2 decay is shortened with increasing Er³⁺ concentration, but changes in luminescence lifetimes are greater for glass samples containing lower (0.5 mol%) than higher (1.5 mol%) Tm³⁺ concentration (see Table 1). This indicates that processes of energy migration between the same lanthanide ions Ln³⁺-Ln³⁺ dominate the energy-transfer processes from Tm³⁺ to Er³⁺ or Yb³⁺ to Er³⁺/Tm³⁺ ions when activator concentrations are high.

Finally, NIR luminescence spectra were measured for lead germanate glass triply doped with Yb³⁺/Er³⁺/Tm³⁺ and then compared to glass samples co-doped with Yb³⁺/Tm³⁺. The results are presented in Figure 4.

To compare the emission bandwidth, the spectra measured under 975 nm excitation were normalized. The observed near-infrared luminescence band centered at about 1.8 µm corresponds to the ³F₄ → ³H₆ transition of Tm³⁺. Luminescence decays from the upper ³F₄ state of Tm³⁺ ions were also registered. Interestingly, the values of FWHM for samples with low Tm³⁺ content are close to 204 nm (0.5 Tm), 206 nm (0.5 Tm − 0.5 Er), and 269 nm (0.5 Tm − 1.5 Er). The later value for 0.5 Tm − 1.5 Er sample is consistent with previous results (FWHM = 270 nm for band at 1.8 µm) obtained for calcium boroaluminate glass co-doped with Er³⁺/Tm³⁺ [50]. It suggests that our glass system with Yb³⁺/Er³⁺/Tm³⁺ ions is a quite good candidate for broadband emission at 1.8 µm. Further spectroscopic analysis indicates that the emission bandwidth is reduced from 267 nm (1.5 Tm) to 241 nm (1.5 Tm − 0.5 Er) and 233 nm (1.5 Tm − 1.5 Er) in the presence of Er³⁺ ions in glass samples containing higher Tm³⁺ concentration.

The previously published results for Yb³⁺/Tm³⁺ co-doped glass pointed out that the co-doping concentrations of Tm³⁺ and Yb³⁺ should be relatively high to obtain an efficient near-infrared luminescence at 1.8 µm [8]. When the concentration of Yb³⁺ is relatively high (5 mol%) and constant in our all glass samples triply doped with Yb³⁺/Er³⁺/Tm³⁺ ions, the excitation energy transfer is favored by processes of energy migration Yb³⁺–Yb³⁺ (²F_5/2,²F_7/2 → ²F_7/2,²F_5/2), Er³⁺–Er³⁺ (⁴I_15/2,⁴I_13/2 → ⁴I_13/2,⁴I_15/2) and Tm³⁺–Tm³⁺ (³H₆,³F₄ → ³F₄,³H₆) with increasing (Er³⁺ and Tm³⁺) activators concentrations. Our experimental observations from luminescence spectra and their decays confirm that the energy-transfer processes depend significantly on both Er³⁺ and Tm³⁺ concentrations. Luminescence decay analysis for samples containing low Tm³⁺ concentration indicates that the ³F₄ lifetime increases from 1440 µs (0.5 Tm) to 1620 µs (0.5 Tm − 0.5 Er) in the presence of Er³⁺ suggesting the energy transfer from erbium to thulium ions and the enhancement of near-infrared emission at 1.8 µm. Then, the measured ³F₄ lifetime decreases to 1229 µs (0.5 Tm − 1.5 Er) with further increasing Er³⁺ concentration. This behavior is related to the increasing role of energy migration Er³⁺–Er³⁺ (⁴I_15/2,⁴I_13/2 → ⁴I_13/2,⁴I_15/2). The enhancement of ³F₄ lifetime in the presence of Er³⁺ is also observed for glass samples containing higher Tm³⁺ content, but this trend is completely different. The measured ³F₄ luminescence lifetime increases from 996 µs (1.5 Tm) to 1260 µs (1.5 Tm − 0.5 Er) and 1527 µs (1.5 Tm − 1.5 Er) in the presence of Er³⁺. It corroborates the results obtained from luminescence spectra. In fact, the intensity of the emission band at 1.8 µm grows with increasing Er³⁺ concentration. In this case, the cross-relaxation processes [³H₄ (Tm³⁺) + ³H₆ (Tm³⁺)] → [(³F₄ (Tm³⁺) + ³F₄ (Tm³⁺)], and [³H₄ (Tm³⁺) + ⁴I_15/2 (Er³⁺)] → [³F₄ (Tm³⁺) + ⁴I_13/2 (Er³⁺)] are enhanced by increasing Tm³⁺ concentration providing an important contribution to the efficient population of the upper ³F₄ excited state and then the improved near-infrared luminescence at 1.8 µm due to the ³F₄ → ³H₆ transition of Tm³⁺ ions. At this moment, it should also be mentioned that the up-conversion luminescence mechanisms including the ground state absorption (GSA) and the excited state absorption (ESA) processes play a significant role in the excited state relaxation between lanthanides in lead germanate glass and should not be ignored. The intensities of NIR emission bands around 1.5 µm (Er³⁺) and 1.8 µm (Tm³⁺) can be diminished by losses of the excited state absorption process (ESA) due to the ⁴I_13/2 → ⁴F_9/2 transition of Er³⁺ ions. It suggests that thulium ions favor the energy-transfer processes between ⁴I_13/2 (Er³⁺) and ³F₄ (Tm³⁺) states by decreasing the mechanism of the ESA process due to the ⁴I_13/2 → ⁴F_9/2 transition of Er³⁺ and consequently the improvement of near-infrared emission at 1.8 µm, independently on single- or dual-wavelength pumping schemes [51]. However, these phenomena will be examined in a separate work.

4. Conclusions

Lead germanate glass triply doped with Yb³⁺/Er³⁺/Tm³⁺ has been examined for near-infrared emission applications. Glass samples were excited at 800 nm and 975 nm. Their emission intensities and lifetimes depend critically on activator (Er³⁺ and Tm³⁺) concentrations. Broadband emission with its spectral bandwidth FWHM equal to 130 nm covering the S+C+L-bands corresponding to the ³H₄ → ³F₄ (Tm³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transitions was measured for glass samples containing 1.5 mol% Tm³⁺ and 0.5 mol% Er³⁺ under 800 nm excitation. The energy transfer from the ⁴I_13/2 (Er³⁺) state to the ³F₄ (Tm³⁺) state and cross-relaxation processes make an important contribution to broadband emissions near 1.8 µm assigned to the ³F₄ → ³H₆ transition of Tm³⁺. The highest emission bandwidth for a glass sample containing 0.5 mol% Tm³⁺ and 1.5 mol% Er³⁺ is close to 269 nm. Based on luminescence decay measurements, the energy-transfer processes, and their mechanisms between the excited states of lanthanide ions in lead germanate glass were confirmed.

Our studies indicate that luminescence decays from the ³H₄ (Tm³⁺) and ²F_5/2 (Yb³⁺) excited states measured for lead germanate glass in the presence of Er³⁺ were shortened compared to Yb³⁺/Tm³⁺ co-doped glass samples. The changes in luminescence lifetimes are greater for glass samples containing low (0.5 mol%) than higher (1.5 mol%) Tm³⁺ concentration. The same effects have been observed for the ⁴I_13/2 lifetimes of Er³⁺. Further investigations revealed that the luminescence lifetimes related to the ³F₄ → ³H₆ transition of Tm³⁺ are enhanced for glass samples with the presence of Er³⁺ ions. It suggests that Yb³⁺/Er³⁺/Tm³⁺ triply doped lead germanate glass is a promising host material for broadband near-infrared luminescence at 1.8 µm. This was discussed based on the energy level diagram including all transitions and processes present in lead germanate glass with Yb³⁺/Er³⁺/Tm³⁺.

Author Contributions

Methodology, J.P., W.A.P., R.L. and W.R.-R.; formal analysis, W.A.P. and W.R.-R.; investigation, J.P. and R.L.; writing—original draft preparation, W.A.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

Publication co-financed by the funds granted under the Research Excellence Initiative of the University of Silesia in Katowice.

Conflicts of Interest

The authors declare no conflict of interest.

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Figures and Table

Figure 1. Normalized NIR luminescence spectra for lead germanate glass containing Yb3+/Er3+/Tm3+ and Yb3+/Tm3+ ions excited at 800 nm (left) and 975 nm (right).

Figure 2. Energy level diagrams for lead germanate glass triply doped with Yb3+/Er3+/Tm3+ ions excited at 800 nm (top) and 975 nm (bottom). All transitions and processes are also indicated.

Figure 3. Luminescence decays from the 3H4 (Tm3+) and 2F5/2 (Yb3+) excited states.

Figure 4. Normalized NIR luminescence spectra (left) and their decays (right) measured for lead germanate glass containing Yb3+/Er3+/Tm3+ and Yb3+/Tm3+ ions excited at 975 nm.

Table 1

Luminescence lifetimes for the excited states of lanthanide ions in lead germanate glass calculated based on decay curve measurements.

Glass Code	Luminescence Lifetime (µs)
	3H4 (Tm3+)	2F5/2 (Yb3+)	4I13/2 (Er3+)	3F4 (Tm3+)
(a) 0.5 Tm	128	201	–	1440
(b) 0.5 Tm − 0.5 Er	103	165	1595	1620
(c) 0.5 Tm − 1.5 Er	53	109	646	1229
(d) 1.5 Tm	72	130	–	996
(e) 1.5 Tm − 0.5 Er	68	134	888	1260
(f) 1.5 Tm − 1.5 Er	62	128	775	1527